Bonding in Metal Alkene Complexes Quiz and Flashcards

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Questions and Answers

What is the reason for the increase in alkene C=C bond length on binding?

The M−alkene σ bond depletes the C=C π bond by donation to M

The C=C π bond is depleted by back donation from the metal and the M−alkene σ bond (correct)

The C=C π bond is strengthened by the M−alkene σ bond

The C=C π bond is strengthened by back donation from the metal

What is the characteristic of the C−H bonds in strongly π basic Pt(0) complexes?

They are unaffected by the metal

They are bent away from the metal to a small extent

They are aligned towards the metal

They are bent away from the metal to a much bigger extent (correct)

What is the effect of electron-withdrawing substituents on carbon on the M-(alkene) bond?

They strengthen the M-(alkene) bond (correct)

They break the M-(alkene) bond

They weaken the M-(alkene) bond

They have no effect on the M-(alkene) bond

What is the difference between the D-C and MCP models?

The D-C model is for weakly π-basic metals while the MCP model is for strongly π-basic metals Signup and view all the answers

What is the length of the C=C bond in free C2H4?

1.337 Å Signup and view all the answers

What is the characteristic of the carbons in an MCP C2H4 complex?

They are rehybridized from sp2 to sp3 Signup and view all the answers

What is the approximate angle that the C1 and C4 substituents twist out of the plane of the ligand?

20–30° Signup and view all the answers

What is the average length of the C1–C2 and C3–C4 distances?

1.46 Å Signup and view all the answers

Which molecular orbitals dominate the bonding to the metal?

ψ2 and ψ3 Signup and view all the answers

What is the result of σ donation to the metal and back donation from the metal on the C1–C2 and C2–C3 distances?

C1–C2 is lengthened, C2–C3 is shortened Signup and view all the answers

At which carbon atom can protonation occur?

C1 Signup and view all the answers

What is the result of M–L binding on the ligand HOMO and LUMO?

The HOMO is depleted, the LUMO is populated Signup and view all the answers

What happens to the metal in a fluxional η1-Cp group?

It rapidly moves around the ring. Signup and view all the answers

How many MOs are formed from the five carbon p orbitals in the MO scheme of Fig. 5.3?

Five Signup and view all the answers

Which metal orbital interacts with ψ1 in Fig. 5.3b?

dz2 Signup and view all the answers

Why is the Cp group not a good π acceptor?

It is a good π acceptor due to the anionic charge. Signup and view all the answers

What must be considered in the MO diagram for a Cp2M metallocene?

The sum and difference of each MO. Signup and view all the answers

What is the effect of the anionic charge on Cp complexes?

They become basic. Signup and view all the answers

What is the purpose of using Et3O+ in complexes?

To form a volatile by-product Signup and view all the answers

What is the characteristic of the η8cyclooctatetraene (cot) hydrocarbon?

Antiaromatic 8π electron Signup and view all the answers

What type of complexes can early metals form with η8-C8H8?

η8-C8H8 complexes Signup and view all the answers

What is the term used to describe fragments that are very dissimilar in composition but have similar frontier orbitals?

Isolobal Signup and view all the answers

What is the characteristic of the frontier orbitals of isolobal fragments?

Similar symmetries and electron occupations Signup and view all the answers

What is the symmetry of the pair of ψ1 orbitals from each ring?

a1g Signup and view all the answers

What is the result of replacing one CH of benzene by a metal fragment isolobal with CH?

A planar MC5 ring without alternating CC bond lengths Signup and view all the answers

Which metal orbital does the opposite combination of ψ1 orbitals interact with?

pz Signup and view all the answers

What is the pattern of d-orbital splitting in Cp2M?

Two-above-three pattern Signup and view all the answers

Which metallocenes are the most stable?

Group 8 metallocenes Signup and view all the answers

What is the result of the bonding and nonbonding orbitals being exactly filled in Cp2Fe?

The antibonding orbitals are empty Signup and view all the answers

Why is the MCp2 unit intrinsically stable?

The reason is not specified in the text Signup and view all the answers

Study Notes

Experimental Structures

Experimental structures of metal-alkene complexes can fall between two extremes: Dewar-Chatt-Duncanson (D-C) model and Metal-Centered (MCP) model.
The D-C model fits best for intermediate oxidation state late metals, such as Zeise's salt, while the MCP model applies for Pt(0).
Both models are considered η2 structures.

Metal-Alkene Bonding

The alkene C=C bond length increases on binding due to two reasons:
- M-alkene σ bond depletes the C=C π bond by donation to M, slightly weakening and lengthening the bond.
- Back donation from the metal lowers the alkene C-C bond order by filling C=C π*.
For weakly π-basic Pt(II), the D-C model results in a slight reduction in bond length, while for strongly π-basic Pt(0), the MCP model applies, leading to a longer C=C bond length.

η2 Complexes

Electron-withdrawing substituents on carbon encourage back donation and strengthen the M-alkene bond.
The bonds to the four substituents of the alkene are bent away from the metal to a small extent in the D-C case, but to a much bigger extent in an MCP complex.
The C1 and C4 substituents twist 20-30° out of the plane of the ligand and bend back to overlap better with the metal.

Diene Complexes

The butadiene frontier orbitals, ψ2 (HOMO) and ψ3 (LUMO), dominate bonding to the metal.
The MO diagram shows that both depletion of electron density in ψ2 by σ donation to the metal and population of ψ3 by back donation from the metal lengthen C1-C2 and shorten C2-C3.
Protonation can occur at C1 where the HOMO, ψ2, has its highest coefficient.

Isolobal Replacement and Metallocycles

The chemical character of a fragment depends on the symmetry and electron occupation of the frontier orbitals.
Fragments that are very dissimilar in composition can have similar frontier orbitals, known as isolobal fragments.
Isolobality helps in understanding metallabenzenes, in which we replace one CH of benzene by a metal fragment isolobal with CH.

η8 Ligands

The antiaromatic 8π electron, nonplanar hydrocarbon, η8-cyclooctatetraene (cot), can form complexes as the reduced, aromatic 10πe cot2− dianion.
Early metals that need many electrons to achieve an 18e structure can also give η8-C8H8 complexes.

Cyclopentadienyl Complexes

The η1-Cp group can be fluxional, in which case the metal rapidly moves around the ring to make all the protons equivalent.
The MO scheme for M-C5H5 shows that the five carbon p orbitals lead to five MOs for the C5H5 group.
The most important overlaps are ψ1 with the metal dz2, and ψ2 and ψ3 with the dxz and dyz orbitals.

Metallocenes

The MO diagram for a Cp2M metallocene requires consideration of both Cp groups.
The bonding scheme retains both the L→M direct donation and the M→L back donation, as well as a d-orbital splitting pattern that broadly resembles the two-above-three pattern characteristic of an octahedral crystal field.
The MCp2 unit is intrinsically stable, and the same structure is adopted for numerous first-row transition metals, even when this results in a paramagnetic, non-18e complex.

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Description

Learn about the bonding and structural properties of metal-alkene complexes, including the D-C and MCP models. Understand how the alkene C=C bond length changes upon binding with metals.

Metal-Alkene Complexes